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Bài `3:`

`a. (2+x)(2-x)(4+x^2)`

`=(2^2-x^2)(4+x^2)`

`=(4-x^2)(4+x^2)`

`=4^2 -(x^2)^2`

`=16-x^4`

`b. (x^2-2xy +2y^2)(x-y)(x+y)`

`=(x^2-2xy+2y^2)(x^2-y^2)`

`=x^4 -x^2y^2 -2x^3y +2xy^3 +2x^2y^2 -2y^4`

`=x^4 +x^2y^2 -2x^3y +2xy^3 -2y^4`

`c. x(x-4)-(x^2-8)=0`

`x^2-4x -x^2+8=0`

`(x^2-x^2)-4x+8=0`

`-4x+8=0`

`-4x=0-8`

`-4x=-8`

`x=2`

Vậy `x=2`